The present invention relates generally to metallic shape memory gaskets, particularly those having one or more components made of material that possesses super-elastic properties suitable for high-performance industrial applications that involve differing ranges of operating temperatures and component materials being sealed.
This application is an improvement of the invention shown in Applicant""s co-pending Ser. No. 09/311,938, filed May 14th, 1999 and entitled, xe2x80x9cStress-Induced Sealxe2x80x9d, the entire disclosure of which is expressly incorporated by reference herein and relied upon
The use of metallic shape memory material, such as Nixe2x80x94Ti (nitinol) and other bi- or tri-metal alloys, has been documented in a variety of technical applications, including fasteners, connectors, clamps and seals. Many such uses have required temperature in order to activate the shape memory material and change its physical state, while others have used mechanical forces that impart stress to cause a super-elastic physical deformation in the material. Still others have employed a combination of temperature and mechanical stress causing a shape memory effect to function in a desired product. As will be seen, however, not all shape memory materials are pseudo-elastic, because not all exhibit super-elastic behavior at their operating temperatures. These various prior approaches are detailed in the references discussed herein. Of particular concern to the instant inventor is the applicability of this technology to gaskets. The use of non-corrosive, metallic shape memory material offers a decided advantage in high performance gasketing environments, versus pliable synthetic rubber or silicone materials, in its ability to withstand a more substantial sealing load under extremes of pressure and temperature while not drying up the way rubbers and silicones can.
U.S. Pat. No. 4,896,955 to Zider and Krumme, entitled xe2x80x9cEyeglass Frame Including Shape Memory Elementsxe2x80x9d, characterizes the use of nickel-titanium shape memory alloys that have super-elastic or pseudo-elastic properties. In criticizing these materials, it is said that the effective useful temperature range for such purely pseudo-elastic components in eyeglass frames is too narrow. This limitation was further said to be remedied by fabrication of such components from materials having xe2x80x9coptimized elasticityxe2x80x9d properties, that is, work-hardened shape memory alloys. In U.S. Pat. Nos. 5,395,193 and 5,584,631, also co-invented by Krumme, the aforementioned optimized elastic materials are further incorporated in a Belleville fastener said to be useful in eyeglass frames. However, Krumme makes no mention of a gasket nor can the same be fairly implied from its disclosure. U.S. Pat. No. 5,683,404 to Johnson, entitled xe2x80x9cClamp and Method for its Usexe2x80x9d, further discusses shape memory materials that are xe2x80x9cpseudo-elasticxe2x80x9d, defining these materials in terms of their ability to exhibit super-elastic/pseudo-elastic recovery characteristics at room temperature. Such materials are said to deform from an austenitic crystal structure to a stress-induced structure postulated to be martensitic in nature, returning thence to the austenitic state when the stress is removed. The alternate crystal structures described give the alloy super-elastic or pseudo-elastic properties. Poisson""s Ratio for nitinol is about 0.3, but this ratio significantly increases up to approximately 0.5 or more when the shape memory alloy is stretched beyond its initial elastic limit. It is at this point that stress-induced martensite is said to occur, i.e., the point beyond which the material is permanently deformed and thus incapable of returning to its initial austenitic shape. A special tool is employed by Johnson to impart an external stretching force that deforms the material which force is then released to cause the material to return to its original condition. While the device is stretched, a member is captured by it and securely clamped when the stretching force is released. This activation of the shape memory component is by application of ail external force to that component, rather than activation of one component by another. Another use envisioned by the aforementioned inventor is to connect the modular components of a medical device, as described in U.S. Pat. No. 5,858,020, by subjecting a component made of shape memory material to an external, i.e., stretching stimulus. A binding or strap device is described in U.S. Pat. No. 5,766,218 to Arnott, ostensibly to provide compressive force via a tensioning loop member of shape memory material. Such clamping operations as noted immediately above require a special tensioning tool and means on the device to attach the tool and impart a stretching/tensioning force. Moreover, none of these contemplate a gasketing operation.
U.S. Pat. No. 5,190,546 to Jervis discloses insertion into a bone cavity of a member made of shape memory material. The walls of the bone cavity are said to impart a compressive force on the member due to interference fit, to cause it to deform and be held in tension within the cavity. The ability of bone matter to withstand such an insertion force without cracking seems highly improbable; indeed, there is no evidence presented that such a result does not occur in the body. Nor is any gasketing function contemplated.
Others have sought to utilize the properties of shape memory materials as locking, connector and bearing elements, e.g., U.S. Pat. Nos. 5,507,826 to Besselink, et al., 5,779,281 to Kapgan, et al., and 5,067,827 to Arnold, respectively; however, such approaches have required temperature to be applied during use. U.S. Pat. Nos. 5,277,435 to Kramer, et al. and 5,876,434 to Flomenblit, et al. similarly relied upon temperature to activate the shape memory effect. Such dependence on extrinsic activation by temperature introduces an added process step and may further be disadvantageous in certain other applications, e.g., seals, where temperature extremes are ordinarily encountered during conditions of use.
U.S. Pat. No. 5,226,683 to Julien, et al., entitled, xe2x80x9cReusable Metallic Seal Using Memory Metalxe2x80x9d, employs a continuous annulus of binary metal or super-elastic material which can be compressed between two surfaces to create a seal. However, the surfaces being sealed must have a yield strength higher than the seal so that it can deform into the surface irregularities creating the seal.
None of the above-mentioned prior approaches have contemplated the formation of an effective gasket such as envisioned by the present inventor. Moreover, a key feature of certain gaskets is their ability to accommodate changes of differing kinds in the members being sealed, such as thermal expansion. Pliable rubbers and silicones, for example, have this characteristic; however, the operating demands are decidedly different in high performance, metallic gasketing applications.
Accordingly, it is an object of the invention to form a durable metallic, non-corrosive gasket, beginning with a shape memory material in its austenitic state and inducing a super-elastic behavior via stress-induction to form an effective high performance gasket. Moreover, it is an object to provide a gasketing assembly wherein there is minimal marring or scoring of the components.
It is a further object to form a gasket that decreases the temperature sensitivity of a component made from a shape memory material. Conversely, it is an object to form a stress-induced gasket using a pre-tensioned shape memory alloy that adjusts for differences in thermal coefficients of expansion or contraction of dissimilar materials comprising those components being gasketed.
A gasket assembly of the present invention has at least two surfaces being sealed and a gasket member made of a shape memory alloy, wherein the gasket member bends to effect a seal between the first surface and the second surface as they are urged together. Preferably, the bend generates a spring force in the gasket, causing the seal. It is further preferred that the top and bottom surfaces of the gasket are continuous, although in an alternative preferred embodiment they are discontinuous.
In a preferred embodiment, means are provided for imparting a compressive force urging the first and second members into respective sealing engagement with the gasket member. Preferably, a fastener imparts the compressive force.
In another preferred embodiment, the gasket member is a washer, more preferably a flat washer.
In another preferred embodiment, the gasket member is a Belleville washer.
In yet another preferred embodiment, the shape memory alloy of the gasket member is super-elastic and, preferably, elastically reversible for reuse.
A process for forming a seal assembly of the present invention includes the steps of providing a first member and a second member with sealing surfaces, also providing a gasket member made of a super-elastic alloy. The process further includes the step of applying a compressive force to one of the members, wherein the gasket member super-elastically bends to effect a seal between the first and second members.
It is preferable in the process to provide the gasket with top and bottom sealing surfaces that present continuous sealing surfaces, respectively.
It is again preferable in the process to provide a means for imparting a compressive force urging the first and second members into respective sealing engagement with the gasket member. Preferably the force is applied by using a fastener.
An advantage of the present invention is the formation of a high-performance, non-corrosive metallic gasketing assembly from a shape memory material that can be activated in a variety of uses by mechanical stress in a relatively simple operation, while in its austenitic state, with a minimum of sealing surface contact that further avoids marring the metallic components.
Another advantage is a stress-induced gasketing assembly using a pre-tensioned shape memory alloy that adjusts for differences in hardness, and of thermal coefficients of expansion or contraction of dissimilar materials comprising those members being gasketed.
Yet a further advantage of the present invention is a high-performance gasketing assembly wherein the gasket bends or xe2x80x9cgivesxe2x80x9d, moving out of the way to form the seal. The result is that the members being sealed are not marred/scored by the gasket. This allows the gasket to possess hardness greater than the members sealed.
A still further advantage is that the present invention forms a gasketing assembly primarily through bending via edge contact of the gasket with the sealing surfaces of the members.
Other objects and advantages will be appreciated by those skilled in the art, by resort to the appended Drawings having reference numerals that correspond to the ensuing Detailed Description wherein the following Figures are further elucidated.